The invention relates to a transparent conducting layer using tin oxide and/or indium oxide, as well as a method for manufacturing such a conducting layer.
All kinds of passive displays such as electrochromic displays, electrolytic displays and liquid-crystal displays require transparent conducting electrodes, which purpose is in general served by glass provided with transparent conducting layers. The known conducting layers are predominantly made of tin oxide and/or indium oxide. In electrochromic and electrolytic displays, the display indication comes about by electrochemical reactions. The conducting layers can participate in these reactions and it is thereby possible to destroy the conducting layers. This can be done particularly by electrochemical reduction of the oxides contained in the conducting layers.
In field-effect liquid-crystal displays, an electric field causes the desired display indication. Accordingly, no detrimental reactions would be expected if liquid crystals were ideal insulators. However, since liquid crystals have a finite resistance, electrochemical reactions occur also in these displays. These reactions occur, within the displays, at the boundary surface electrode/liquid crystal. They moreover usually occur at the edges of the segments as a result of the high field strength producing disorientation of the edges and discoloration of all the transparent electrodes, caused by metallic tin or indium.
Outside the actual display device, a water film may be formed under unfavorable ambient conditions, and capillary condensation can occur particularly in cracks. Since the water film is conducting because of impurities in the air, this creates conditions for the occurrence of electrochemical reactions which can lead to the destruction of the conductors which consist of tin and/or indium oxide. These conductors which are connected to the individual segments and symbols of the display are required for driving.
The destruction of the conductors can occur in many kinds of displays. It has been attempted to delay this process or to suppress it altogether, particularly by using especially resistant materials. In addition, care is taken that there are no cracks in which capillary condensation can take place. However, all these measures are expensive and do not lead to the desired success. Another measure for preventing electrochemical reactions, namely, covering the conductors with a protective varnish is only partially possible because of the contacts which have to be made with the conductors. This preventive measure is therefore effective only to a limited degree.
Attempts have been made to prevent the destruction of the electrodes which takes place inside the display devices of liquid-crystal displays by using a sufficiently high driving frequency. These attempts, however, presuppose that the electrode reactions are reversible. These attempts utilize addition to the liquid crystals of small amounts of an oxidizable or reducible electron donor-acceptor complex (see German Offenlegungsschrift DE No. 30 42 517 Al, claim 1 or the corresponding U.S. patent application Ser. No. 318,613, of Nov. 5, 1981) or use liquid crystal components with electron donor-acceptor properties directly (see German Offenlegungsschrift DE No. 31 25 758 Al, claim 1). By the mentioned measures, the electrode polarization with the current given by ambient influences can be reduced, and thereby the prerequisite is created that the reaction at the electrodes caused by this current are reversible.
The mentioned measures are so effective that even d-c operation of liquid-crystal displays becomes feasible. If additives are used, it cannot be precluded entirely that an impairment of the liquid crystals can take place in certain individual cases. On the other hand, the choice of liquid crystals with electron donor-acceptor properties is still limited at the present time.
In electrochromic and electrolytic displays, methods to prevent the destruction of the electrodes have been studied which cover the electrode edges with a chemically inert layer (see U.S. Pat. No. 3,836,229). Such a measure is very expensive, however, from a production point of view.
It is, therefore, an object of the invention to develop a transparent display material having a conducting layer of a tin and/or indium oxide composition which withstands electrochemical attack and does not need elaborate measures such as those mentioned above.